How to kill it? I'm assuming by just killing the process. The actual trick is letting your virus/trojan/RAT/what-have-you be undetected whilst trying to shut it down.

Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning. -Rick Cook

I'm guessing the OP is trying to have the trojan sent as a payload, have the avira killed then execute the trojan. Most likely you can't just kill the process and be good. You have to think about admin privs and the program then asking you to confirm by clicking.

In that case you also have to look at the anti-virus's methods of persistence and analysis. Many AVs use some method such as having an av check when an executable is run, or when a file is made/changed, etc.